Reverse bimetal thermostat



April 7, 1959 I A. SCHWANEKE REVERSE BIMETAL THERMOSTAT Filed March 17,1954 2 Sheets-sh ed 1 55 5 INVENTOR.

United States Patent "ice 2,881,300 REVERSE BIMETAL THERMOSTAT Alfred E.Schwaneke, Northbrook, Ill., assignor to A-M Corporation, Chicago, 111.,a corporation of IlllllOiS Application March 17, 1954, Serial No.416,884 4 Claims. (Cl. 219-25) The present invention relates tothermostats employing bimetal blades, and more particularly tothermostats which are subjected to variable settings in householdappliances which have rapid warmup and recovery cycles.

By way of example, it has been found with conventionally equippedthermostatically controlled electric irons for household use that theinitial warmup objectionably overshoots the set temperature regardlessof whether the setting is a high or a low one. However, in the eventthat the setting is low, danger of scorching the fabric being ironed isa very great one if the houswife used the iron right away. In manyinstances, an iron set for 425 F. has heretofore reached a temperatureas high as 600 in seven minutes on the initial warmup.

In order to overcome this, it has been proposed to use a construction inwhich a portion of the thermostat which controls the switch is reversedwith respectto the other portion as held together by a buttweld or acrush weld. Such a construction is sometimes referred to as a reversedthermostat metal in which the two pieces of thermostat metal weldedtogether have their high and low ex pansion sides reversed.

Although this expediency has met with some success in reducing theovershoot, another problem has been incurred with these thermostats.There is a lack of uniformity between thermostats so constructed whichotherwise visually appear to be exactly alike. Even though sheets fromwhich reversed bimetal blades are made are trimmed and rolled afterwelding, a great deal of care and accompanying cost is necessary in allthe details of the welding operation to produce a predictable weld, yetthe welds vary from sheet to sheet with respect to the operationalcharacteristics of the blades produced. More- CV61, subsequentoperations in finishing to the desired thickness require such attentionthat reverse metal therm'os'tats are not successfully made upon aproduction scale basis without 100% test inspection. Inspections,

tests, and other time-consuming endeavors require expensive labor andincur great waste in discards to obtain commercial thermostats which arein any way acceptably alike in their performance throughout the range ofadjustments which will be required of them.

Not only this, but not all types of thermostat metals lend themselves toconventional reverse welds, and unless the two components of the bimetalstrip have similar properties such as electrical resistivity, chemicalcompositions and melting points, the weld is not a strong one nor onethat will be capable of sustained operations under the spring tensionsof the switch elements involved in the working of a thermostat. In somecases the thermostat metal has to be given a preliminary treatment inorder for the two otherwise dissimilar metals to provide any kind of abond at the joint; and, although the rolling operation after the weldvery often exposes the existence of defects so that rejection can be hadbefore a further processing of the bimetal blades, yet other defects arenot dis covered with the final testing stage after assembly of thecomplete thermostat.

Patented Apr. 7, 1959 It is oneof the objects of the present inventionto pro vide a reverse thermostat metal blade which avoids thedifliculties mentioned, yet is simple to make and is uni form in itsoperation to control or eliminate overshoot, an overshoot of not toexceed 10 to 20 F. being desirable in some appliances.

A further object of the invention is to provide a reverse thermostatmetal characterized by a rapid on and 05" cycle between narrow constantlimits at high temperature settings and with less frequent on and offcycling between narrow constant limits at low temperature settings.

The invention is further characterized by a reverse thermostat metalblade which maintains its characteristics throughout its expected life.

Another object of the invention is to utilize identical ness of metalstructure at the welding joint of a reverse thermost metal without needof rerolling the bimetal after welding is accomplished. I

Another object of the invention is to provide for rapid heatconductivity between the two sections of a reverse thermostat metalblade.

The invention is further characterized by a reverse thermostat metalblade construction which is easy to make, whose tolerances are simple tocontrol, and whose operation is uniform throughout long production runs.

These being among the objects, other and further ob jects will becomeapparent from the drawings and the description relating thereto.

In the drawings, Fig. 1 is iron in which a preferred bodied;

Fig. 2 is a top plan view of the iron shown in Fig. 1;:

, Fig. 3 is a perspective view of a thermostat partly in sectionemployed in the iron shown in Fig. l, in which is embodied the preferredform of the bimetal element;

Fig. 4 is an enlarged side elevational view of the bi= metal bladeemployed in the thermostat shown in Fig. 3', the bimetal element beingshown in its cool condition;

Fig. 5 is a view similar to that in Fig. 4, showing the relativedeflection of the bimetal during the initial warmup stage;

Fig. 6 is a view similar to those in Figs. 4 and 5 showing the bimetalblade in the shape assumed by it after the warmup period has beenaccomplished;

Fig. 7 is a section taken upon line 77 in Fig. 6;

Fig. 8 is a section taken upon line 8 -8 of Fig. 7;

Fig. 9 is a section taken upon line 99 of Fig. 7; and

Fig. 10 is a performance chart of the thermostat illus trated in Figs. 3to 9 inclusive.

Generally speaking, for a quick understanding of the invention, it canbe said that the invention finds an embodiment in transversely cuttinginto two pieces a bimetal blade intermediate its ends; turning the onepiece over to lay it on the other piece in partial overlappingrelationship at the cut edges; and, welding the two pieces together atpoints spaced crosswise of the blade within the overlap area. Theunwelded portions of the overlap serve as plane to plane contact areasheld in face-to-face relationship under pressure contact to serve asorienting guide surfaces that preserve a pre= determined relationshipbetween the blade pieces, before, during and after the weld isaccomplished. Thus, uni formity is maintained and a wide area isprovided for rapid heat exchange conductivity between the two ele'ments.

More particularly, an electric iron a side elevation of an electric formof the invention is ear 10 is shown in I Fig. 1, having a heater 11embeddel in the sole plate 12 which is energized with house currentthrough the con venience or extension cord 13 plugged into the socket 14at the rear end of the iron. Electrical current through the heater 11 iscontrolled by a thermostat 14- that it connected in series therewith andis mounted in heat exchange contact with the sole plate 12 between thesides of the U-shaped heater 11K The control shaft 15 of the thermostatextends above the top of the housing 16 where itlreceives a handle 17within finger reach below the main handle 18 of the iron.

.1 Referring to Fig. 3, the thermostat 14 comprises a mounting bracket20 having ears'21 on the opposite sides thereof apertured as at 22 formounting the thermostat within the iron housing 16. At one end of thebracket 20, a sleeve rivet 23 is received which carries on itinsulating,washers 19 made of ceramic which space from each other inelectrically insulated relationship an upper contact carrying spring 24stressed to spring downwardly and apertured to receive therethrough aceramic contact 25 carried on the bottom end of the adjustment shaft 15;asecond contact carrying spring 26 disposed below the first and stressedupwardly to ride against the ceramic pin125 on the lower end of theadjustment screw 15 in all positions of its adjustment. Below these twocontact carrying springs is mounted the thermostat metal blade indicatedgenerally at 27 which carries on its outer end a metal cup 28 receivingin supported relationship a headed ceramic pin 30 which engages theupper spring 24 beyond the contact engagement of the two springs toforce the upper spring 24 upwardly when the bimetal member 27 issubjected to increasing heat. The two contacts will remain in electricalcontact with each other except when either the adjustment shaft memberor the bimetal blade member forces its respective contact beyond thepoint where the other contact can follow as controlled by the othermember.

\ Referring to Fig. 4, the bimetal 27 is structurally made of two layersor laminations of metal bonded together at their interface of differentcoetficients of heat expansione One of the layers 31, and in thisconnection the numeral 31a indicates the same layer, constitutes thelayer of low coeflicient of expansion and preferably comprises ironhaving 42% nickel. The other layer 32, and 32a, constitute the layer ofhigh coefficient of expansion, and preferably is ironhaving the contentof 17% nickel and 2% chrome. The blade 27 in its initial production comprises a single stamping, preferably having parallel sides an edge lapestablished as at 38 at their sheared ends with one of the layers havingthe same 'coelficient of expansion in both pieces in contact inface-to-face relationship. In the particular embodiment shown in Fig. 4,the layer placed in face-to-face contact is the one made of iron havinga 42% nickel content to provide the low coeflicient of expansion.

1 With the two sections 36 and 37 so disposed, three spaced welds aremade as indicated in Fig. 7 at 40 to hold the overlapping portionstogether at 38, leaving two areas of the contacting faces inface-to-face contact as at 41 under pressure exerted by the welds 40.These face-to face unwelded areas held together under pressure preserveand maintain the relative planar relationship betweenthe two thermostatsin their resting stage as shown in Fig. 4. Moreover, this area 41 is ofsubstantial expanse, sufliciently so to provide for a large amount ofheat. transfer conductivity between the two elements 36 and 37 inaddition to the intimate relationship established at the welds 40.

6 Moreover, the pressure contact between ends as at 41 is suflicientlyrugged and constant that the load imposed on the thermostat at thebutton 30 is sustained and carried by the thermostat with greatlongevity andiwih uniform results, the two sections dishing as at 42{Fig 6) with reversed curvature to give third di-' the overlappingmension strength and the welds 40 holding the two to low temperaturescontrolled gether and the faces 38 supporting them before, during andafter the welding in an exact predictable and controlled relationship.

It is immaterial whether the section 37 is overlapped on top of or belowthe section 36, the same predeterminable standards can be maintained, itbeing preferred in the embodiment illustrated to have the iron sectionscontaining 42% nickel adjacent each other. Moreover, because of the factthat the contacting layers are identical metals, any welding ofconventional thermostatic metals can be accomplished easily and with thecomparable re sults within the scope of the present invention, the coefficients of expansion being the only variable with bimetals ofdifferent metallurgical characteristics.

However, in the particular embodiment illustrated where the thermostatis intended to be used with an electric iron, the layer of high heatexpansive metal indicated at 32 is preferably on the bottom and disposedin close, if not in intimate heat exchange contact with the sole plateof the iron. Then in operation, when the heat is first turned on, heatis conducted first into the section 27 of the bimetal as shown in Fig. 5with the other section 37 remaining substantially straight. Thus,flexure of the section 36 will operate through the straightness of thesection 37 to dispose the top of the ceramic button 30 well above thepoint at which it normally breaks contact between the two switchcontacts. In other words, the straightness or heat lag of the element 37will cause the contacts to be separated before the normal heat, to whichthe section 36 is exposed, has reached the section 36 to establish itsoperating flexure at any given setting of the screw 15.

After the first opening of the contacts by the button 30 when thethermostat is flexed as shown in Fig. 5, further heat present in theiron is conducted to the section 37 which causes it to bend in areversed direction to the form represented'in Fig. 6. Thereafter, therelative fiexure of both elements or sections 36 and 37 cooperate indetermining the location at which the button 30 will break the contactsof the switch. In this connection, it should be noted that the locationlengthwise of the bimetal blade of the line of severance 35 controlsovershoot or under- The longer the element 36 is, the greater theovershoot, whereas the shorter the element 36 is, relatively speaking,undershoot conditions can be attained. However, once the location of theline of severance is determined for performance and results, uniformperformtion techniques.

mounting hole is each The warmup period is indicated at 50 to a settingof and provides a desired 10 overshoot at 51. Thereafter follows a makeand break heat control which varies less than 20. Successive highersettings were then made with the results as shown. In each, theovershoot was 10 or less and the thermostat held within 20 range on eachsetting. Thereafter, the iron was cooled off and full range setting warmup Was instituted as at 53. There was no overshoot on the warm up and noundershoot when the iron was subjected to a load and a lower resettemperature.

It will be noted that the differential between high and by thethermostat is substantially constant at all levels, indicating that thethermostat normalizes itself upon the first ofl portion of the cyclefollowing the new setting, and thereafter performs its function with amarked evenness of cycles. A further and unexpected result with thethermostat is found at the higher temperature settings where thedifferential between the high and low is about the same, the cyclingWith a blade wide.

of the thermostat is increased in frequency, maintaining a temperaturein the sole plate which is substantially constant at all temperatures.Moreover, thermostat after thermostat produced as described duplicatethe characteristics shown.

Having thus described the preferred embodiment of the invention where aslight overshoot of less than F. is desired and the structure whichenables mass production within the objects stated, it will be readilyapparent to those skilled in the art how various changes andmodifications can be made structurally within the teachings andprinciples of the invention disclosed, the scope of which iscommensurate with the appended claims.

What is claimed is:

1. In combination with an iron equipped with a sole plate having aheater embedded therein, a thermostat connected in series with saidheater and comprising a mounting bracket fixedly secured to said iron, apair of spring blades carried at one end thereof by said bracket andextending outwardly therefrom in facing relation and being equipped attheir outer ends with contact points, said spring blades being biasedtoward each other for engaging said contact points to establish anelectric connection therebetween, one of said spring blades beingprovided with an opening therethrough intermediate the ends thereof, anadjustable control member mounted upon said bracket and having a portionthereof extending through said opening and into engagement with theother of said spring contacts to provide positional selectivity for thelocation thereof, and a bi-metallic element secured at one end to saidbracket and extending in facing relation with said spring blades andbeing equipped at its other end with a non-conductive pin engageablewith the opening-equipped spring blade to move the same away from theother spring blade for separating said contact points, said bi-metallicelement comprising a pair of metal strips secured together throughoutthe length thereof in laminated juxtaposition and having differentcocfiicients of thermal expansion, said strips being severedtransversely intermediate the ends thereof to define two bi-metallicsegments oriented with the severed ends thereof one above the other inoverlapping relation, one of said segments being inverted with respectto the other to position the identical metal laminations thereof inadjacency, and said overlapping ends being compressed at transverselyspaced points thereacross and welded together at such points to unitesaid segments into the aforesaid bi-metallic element, the relativeeffective deflection lengths of said segments as established by theextent of the relatively non-defiective overlapping areas thereof beingdeterminative of the degree of the overshoot and undershoot afforded bysaid bi-metallic element in its control operation and said segmentshaving deflection lengths that are approximately equal, the segment ofsaid bi-metallic element having the end thereof mounted upon saidbracket being oriented so that the metal lamination having the greatercoefficient of thermal expansion is disposed proximate to said soleplate and heater therein.

2. In combination with an iron equipped with a sole plate having aheater embedded therein, a thermostat connected in series with saidheater and comprising a mounting bracket fixedly secured to said iron, apair of spring blades carried at one end thereof by said bracket andextending outwardly therefrom in facing relation and being equipped attheir outer ends with contact points, said spring blades being biasedtoward each other for engaging said contact points to establish anelectric connection therebetween, one of said spring blades beingprovided with an opening therethrough intermediate the Cir ends thereof,an adjustable control member mounted upon said bracket and having aportion thereof extending through said opening and into engagement withthe other of said spring contacts to provide positional selectivity forthe location thereof, and a bi-metallic element secured at one end tosaid bracket and extending in facing relation with said spring bladesand being equipped at its other end with a non-conductive pin engageablewith the opening-equipped spring blade to move the same away from theother spring blade for separating said contact points, said adjustablecontrol member being calibrated in temperature increments whereby saidhimetallic element functions to separate said contact points when thetemperature of said sole plate approximates that for which the controlmember is set, said bi-metallic element comprises a pair of metal stripssecured together throughout the length thereof in laminatedjuxtaposition and having different co-efficients of thermal expansion,said strips being severed transversely intermediate the ends thereof todefine two bi-metallic segments oriented with the severed ends thereofone above the other in overlapping relation, one of said segments beinginverted with respect to the other to position the identical metallaminations thereof in adjacency, and said overlapping ends beingcompressed at transversely spaced points thereacross and welded togetherat such points to unite said segments into the aforesaid bi-metallicelement.

3. In a thermostat structure characterized by being initially adjustableto provide selective degrees of overshoot and undershoot in a controloperation, a bi-metallic element comprising a pair of metal stripssecured together throughout the length thereof in laminatedjuxtaposition and having different coeflicients of thermal expansion,said strips being severed transversely intermediate the ends thereof todefine two bi-metallic segments oriented with the severed ends thereofone above the other in overlapping relation, one of said segments beinginverted with respect to the other to position the identical metallaminations thereof in adjacency, and said overlapping ends beingcompressed at transversely spaced points thereacross and welded togetherat such points to unite said segments into the aforesaid bi-metallicelement, the relative effective deflection lengths of said segments asestablished by the extent of the relatively non-deflective overlappingareas thereof being determinative of the degree of overshoot andundershoot aflorded by said bi-metallic element in its control operationand said segments having deflection lengths that are approximatelyequal.

4. The thermostat structure of claim 3 in which one end of saidbi-metallic element is fixedly mounted and the other end thereof isequipped with a non-conductive pin, and in which the segment of saidbi-metallic element having the mounted end thereof is oriented so thatthe metal lamination thereof having the greater coetficient of thermalexpansion is disposed proximate to the source of heat to be controlledby the thermostat structure.

References Cited in the file of this patent

